(:~:) is from Data.Type.Equality.
data a :~: b where
Refl :: a :~: a
What does this smiley :~: stand for?
On 13.01.2016 06:58, David Feuer wrote:
These are not likely to be *useful*, but they fall easily into the
general bucket of "exactly one sensible instance exists".
instance Semigroup (a :~: b) where
r <> _ = r
instance a ~ b => Monoid (a :~: b) where
mempty = Refl
mappend r _ = r
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Andreas Abel <>< Du bist der geliebte Mensch.
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